The present invention relates to a method and apparatus for measuring the maturity or degree of cell wall thickening of a sample of naturally occurring cellulosic fibre including cotton fibre.
Cellulosic fibre such as cotton typically has a central lumen or hollow region that represents the residual protoplasm of the living fibre cell which has a cell wall. An important property affecting the quality of fibre having the structure of cellulosic fibre is the degree of thickening of the cell wall by cellulose which is sometimes referred to as fibre maturity or circularity.
Fabric manufactures and spinners regard fibre maturity as an important indicator of the suitability of the fibre for processing from both a chemical and a physical perspective.
For example, immature fibre which is fibre with little or no cell wall thickening is known for causing the following problems during processing: small entanglements called neps; irregularities in processed fibre assemblies including finished yarns; and non-uniform in dyeing of fabrics.
More generally immature fibre decreases processing efficiency and particular steps may be taken to reduce the processing difficulties depending on the maturity of the fibre.
In addition pressure to manage fibre maturity is increasingly being exerted on research agronomists and plant breeders and therefore there is a need to develop a suitable technique for testing the maturity of cellulosic crops in a farming and harvesting environment.
The measurement of fibre maturity particularly cotton fibre has been the subject of 40 years of research and is still viewed as a difficult technical problem. A technique that has in the past been used for measuring fibre maturity involves the direct measurement of the cross-sections of a fibre using a microscope to determine fibre maturity and is regarded as a benchmark for all other tests. However, this direct technique suffers from significant experimental error due to the microscope measurements involved and the limited numbers of fibres that can be practically measured. Other indirect techniques have failed to generate sufficient industry confidence because of their lack of accuracy and/or precision.
Polarized light microscopy is a technique that has long been used to investigate the crystalline structures of inorganic and inert organic materials, e.g., minerals, fibres (natural and synthetic), bone, china, chitin and some fixed sections of organisms. The technique has been used extensively in textile and industrial fibre identification and particularly of fibres that exhibit birefringent properties, i.e., fibres that behave like a uni-axial optical crystal. The optical axis in birefringent fibres is usually parallel along the fibre axis with the refractive index being dependent upon the plane of polarization of the incident light. When plane polarized light is transmitted through a birefringent object the light ray is split into two mutually perpendicular vibrating fast and slow rays, which propagate through the object at two different speeds. Upon emerging from the object a phase difference occurs between the fast and slow rays. When recombined into a single ray by passage through a second polarizor (analyzer) the rays interfere with each other, which in turn create different interference colours that highlight different crystalline characteristics.
A standard test for determining the maturity of fibres by viewing them through crossed polarizing lenses and a first-order red Selenite compensator plate is described in a text entitled “The Standard Method of Test for Maturity of Cotton Fibres (Sodium Hydroxide Swelling and Polarized Light Procedure), 354-359, Designation: D1442-00, ASTM Textile and Fibre Test Methods 2000”. The compensator plate is inserted between the polarizing lenses to increase the level of retardation between the slow and fast rays and hence improve the intensity of colours produced when the rays are recombined. The compensator is also known as wavelength retardation plate or wavelength filter.
The standard test involves arranging a bundle of fibres parallel to each other with a minimum of overlapping in a solution such as water or a clear mineral oil on a glass microscope slide. A cover slide is then positioned on top of the fibres before being placed between the crossed polar lens arrangement. The interference colours appearing from the fibres are the result of the optical phenomena described above and have been classified in a text entitled “Polarized Light Preferred for Maturity Tests” Textile World, February 1945, by Grimes.
Table 1 below provides the accepted standard interference colours for mature and immature cotton fibres compiled by Grimes.
TABLE 1Colours of cotton fibres under polarized lightWITHOUTWITH SELENITE PLATESELENITEAdditiveSubtractiveFIBREPLATEColoursColoursCLASSIFICATIONFirst OrderSecond OrderFirst OrderMATURElight yellowYellowLight yellowwhiteGreenYellowIMMATUREgray-blueBlueOrange-yellowgrayPurpleOrange
A disadvantage of the standard test is that the operator must make an assessment of the colours of the fibres and make a subjective decision on the colour of the fibres which gives rise to large discrepancies in the results from different laboratories. Furthermore, the test is too slow to be carried out for routine testing applications in terms of both specimen preparation and test time. According to our experience, ordinarily the time period required to carry out the standard test on a sample of fibres is in excess of 30 minutes. There would also be additional time in preparing the specimen prior to testing.
It is an object of the present invention to alleviate the disadvantages of the standard test method described above while measuring the maturity or cell wall thickening of cellulosic fibres including cotton.